Many powerboats are equipped with trim tabs which extend from the rear of the hull and are pivotally attached at a point at the lower rear point of the hull. Trim tabs are well known in the industry for their value in controlling the trim of a boat in view of varying load distributions, speeds, water and wind conditions and other variables. The trim tabs and the associated actuators which move the tabs up and down in the water, typically remain underwater when the vessel is at rest.
The most common type of drive for a trim tab actuator is hydraulic, wherein a hydraulic pump is mounted inside the boat and connected to a hydraulic actuator which is, in turn, connected to the trim tab. Hydraulic pressure moves the trim tab down through the motion of the ram which moves in and out of a cylinder. Upon removal of hydraulic pressure, a spring returns the ram which raises the trim tab. This system is typically very effective when new, but may develop leakage problems after a relatively short time. Typically, the seal fails when the cylinder surface becomes covered with barnacles or other similar abrasive materials. These abrasive materials then scrape against the ram to cylinder seal resulting in leakage of hydraulic fluid out of the cylinder. This is undesirable since even small amounts of oil leaked may cause a sheen on the water and adversely affect the environment and constitute a violation of maritime environmental laws. Also, continued leaking of the system's hydraulic oil will render it inoperative if not constantly refilled.
Similarly, another type of trim tab actuator has a reversible electric motor and screw mechanism mounted within a housing which drives a ram in and out to cause the trim tab to move up and down as desired. If the ram is fouled, as described earlier, and the seal is damaged, water will leak into the housing and the motor and associated mechanism will fail, causing the tab to become useless.
A further contributing factor concerning water leakage into an electro-mechanical trim tab actuator is the fact that when the ram moves outward within the housing, a vacuum is created because it is a closed system experiencing an expanding volume. This vacuum can draw water into the housing, particularly if the seal is already damaged. Even water vapor that is drawn into the housing with each use as a result of the vacuum in the housing may have a corrosive effect on the mechanism in the housing.